In general, in an image forming apparatus of an electrophotographic system, plural photoconductive drums are arranged in parallel and toner images of plural colors formed on the respective photoconductive drums are multiply transferred onto a sheet to obtain a color image. In such an image forming apparatus, a residual toner remains on each of the photoconductive drums. When an intermediate transfer belt is used, a residual toner also remains on the intermediate transfer belt.
Therefore, the residual toners remaining on the plural photoconductive drums and the intermediate transfer belt are removed by cleaning devices such as blades. The removed residual toners (waste toners) are collected in a waste toner box.
In the waste toner box in the past, when the waste toner box is filled with the waste toner, it is detected by an optical sensor that the waste toner box is full. The waste toner box is replaced with a new waste toner box. The waste toner falls from a collection port of the box and piles up below the collection port in a mountain shape. The height of the waste toner in the box is the largest in a portion where the waste toner piles up in the mountain shape. Therefore, the sensor is set in a position where the sensor can detect, before the mountain-like portion reaches the collection port, that the box is filled with the waste toner.
JP-A-2002-149022 and JP-A-2002-149023 disclose image forming apparatuses including waste toner containers.
In some image forming apparatus, a system for performing development while replacing a carrier little by little is adopted. The mountain shape of the waste toner formed in the box is different depending on the content of the carrier in the waste toner. The mountain shape is also different depending on an amount of the waste toner. Therefore, even if the sensor detects that the box is full, an amount of the waste toner in the box may fluctuate.